In recent years all industry has become aware of the failure potential of unit or plant protection (trip) systems. Two important concepts are involved: reliability and security.
The reliability of any system is defined as the ability to deliver a trip signal when required. The security of any system depends upon the ability to prevent spurious or false trips under many adverse conditions.
Although no system can be made immune from equipment failures, a greater degree of reliability can be achieved with redundancy. This can be defined as the introduction of auxiliary elements and components to perform the same function as other elements in the system.
It is well known that the reliability of an unredundant system is a decreasing function of the failure rates of components, of the system size and of the time the system functions. Protection systems are always tending to increase in size and complexity. To maintain system reliabilities at acceptable levels, these increases must be counteracted either by a reduction in failure rates, or by applying redundancy. Impressively large improvements can be achieved by using a fairly small degree of redundancy. This must be compared to the relatively small improvements in unredundant system reliability which can only be produced by large expenditures of time and money in component improvement, testing and inspection.
Redundancy schemes are various and obviously each has its own value, which relates to its extra cost and the increased degree of protection. In this respect, the invention disclosed here offers value which is higher than those offered by conventional and known redundancy schemes.